Every now and then a news story appears that seemingly comes out of nowhere. In the world of structural engineering and construction, these stories are normally issues that hitherto hadn’t been well known.
Enter RAAC — or in technical parlance, Reinforced Autoclaved Aerated Concrete — However, what is RAAC, and why is there such a furore surrounding its use all of a sudden? Is it dangerous? Why is it a problem? Let’s take a closer look.
What is Reinforced Autoclaved Aerated Concrete (RAAC)?
Reinforced Autoclaved Aerated Concrete (RAAC) is a modified form of reinforced concrete. Reinforced concrete, as some may know, is one of the most abundant man-made materials on the planet, valued for its incredible compressive strength. Making it ideal for building purposes. One could think of it almost as like a liquid stone, which sets rock-hard upon curing.
Despite being incredibly useful, concrete’s weight does pose challenges when it comes to design and sustainability. . In the design of reinforced concrete elements, the self-weight of the concrete often represents the majority of weight an element needs to support.
Consequently, it has a significant effect on how strong the element needs to be. The concrete strength is dictated by its depth and the amount of steel reinforcement, thus increasing the volume of material needed. This increases its cost and carbon footprint by increasing its weight — a kind of “Catch 22” situation.
To this end, the idea behind RAAC is, on paper, rather a clever one. So, what is RAAC? Well, it is a method of reducing the weight of concrete by way of introducing air bubbles. This reduction minimises material requirements, benefiting cost and environmental sustainability.
You can get away with adding this air into the concrete, because you don’t necessarily need to mobilise its full compressive strength in all cases – especially when you reduce the weight!!
How To Identify RAAC
In the world of construction, being able to identify the right materials is crucial. When it comes to working out how to identify RAAC, recognising its unique properties is essential. You must keep an eye out for a number of key characteristics that help to distinguish RAAC from traditional concrete. This includes weight, porosity, colour, texture, and much more.
Key characteristic differences between RAAC and traditional concrete that can help you in identification includes:
- Lightweight: RAAC is notably lighter than more conventional concrete. It is less dense to the touch, making it much easier to lift and handle.
- Porosity: You should try to look out for distinctive porous structures with evenly distributed air bubbles. This acts as a hallmark of RAAC.
- Colour: Due to its composition and the incorporation of the air bubbles during its production, RAAC will typically have a lighter colour when compared to regular concrete.
- Sound: If you tap on the surface of RAAC, you should hear a distinctive hollow sound. This is a direct result of its porous nature and is in direct contrast to the denser sound associated with traditional concrete.
So, What is RAAC’s Main Problem?
Concrete excels in compression, but is lacking in tension. To counter this, steel reinforcement is essential. In most applications, tension is present, making reinforcement vital for maximum structural integrity. Any failure in this bond, or any loss of steel strength could lead to significant failure of the concrete element.
Problem One: Steel Reinforcement Corrosion
A properly constructed and well-maintained RAAC panel, that is shielded from environmental exposure to water and pollution, should remain safe and serviceable. The primary concern with this material lies with situations where it is lacking in this protection. In such cases, oxidating compounds such as water can infiltrate the concrete via the matrix of bubbles.
This won’t pose much of a problem to the concrete itself, however it is a severe issue for the steel reinforcement, which is itself pivotal to RAAC’s strength. As steel oxidises (rusts), it expands in volume and weakens. Rust isn’t strong like the steel from which it develops, meaning it weakens the reinforcements while creating stresses in the surrounding RAAC matrix.
Since the RAAC isn’t very good at accommodating these (tension) stresses, it ends up cracking and losing integrity. In some cases it even ends up ’spalling’. This is where the concrete falls away from the steel, further exposing it to oxidising compounds. Consequently, this accelerates rusting, and further breaks the bond between the reinforcement and the concrete.
Problem Two:End Bearing Overlap
One of the biggest concerns surrounding RAAC is the potential for sudden and catastrophic failure, which can occur without warning. This can be attributed to several factors: RAAC’s low intrinsic strength, issues relating to the construction’s quality control process, and the corrosion issue highlighted above.
Sudden and catastrophic failure in structures often occurs as a result of shearing of an element near a support. This rings true for RAAC as well, where an insufficient overlap between the supporting structural element and the RAAC (together with its reinforcement) can lead to a sudden and catastrophic failure.
As mentioned previously, environmental exposure can cause a kind of “debonding” between the RAAC and its reinforcement. Should this occur at a support point, the RAAC would become unable to bear the load on its own, leading to a sudden and catastrophic fracture.
What’s the solution?
With a better understanding of exactly ‘what is RAAC’, it’s important to now consider what solutions there are out there.
Much of the noise in the press recently has surrounded the need to strip out all RAAC with immediate effect. Others say we must demolish and reconstruct all buildings containing RAAC as a matter of urgency. While this may be necessary in the medium-long term, the short term discussion should likely focus on risk, and risk mitigation.
The reason why we say this is because, as we mentioned above, well-protected, maintained, and constructed RAAC shouldn’t in and of itself present any risk. If a building has been standing for 50 years and doesn’t present visual evidence of structural disturbance or degradation, then what is the reason to think that it won’t stand for another 50?
Buildings don’t take much time to demolish. They do, however, take time and a significant amount of cost (both financially and to the environment) to plan and re-build. For that reason, there surely needs to be a short-to-medium term solution to the issue. This solution would, where possible, allow pupils back into schools and patients back into hospitals,reducing additional cost to society.
Stage One: Inspection and Risk Management
As previously discussed, structural failure is often linked to degradation.
Degradation which usually only becomes visible once all finishes are stripped away. For this reason, a visual condition survey of RAAC structures must be carried out as the first step.
The condition of the elements upon inspection should then inform our risk mitigation strategy. If there is no deterioration, then the risk of failure, catastrophic or otherwise, becomes lower.
However, as part of the inspection strategy, we must also seriously consider the risk of failure without warning. The other main contributing factor seems to be not only degradation, but how much the ends of the RAAC element overlap onto supporting elements. This, therefore, must also be visually assessed in accordance with the relevant standards as part of the process.
Once this is confirmed, non-destructive reinforcement detection needs to be carried out in order to assess the reinforcement overlap which can’t be seen with the naked eye. Once both visual and non-destructive assessments have confirmed satisfactory conditions, the risk of failure should be minimised.
Stage Two: Reinforcement (Where Needed)
In situations where the risk is deemed to be too high, or where either the RAAC end bearing or reinforcement requirements aren’t met (as per the stage one inspection), then reinforcement should be considered.
This could take the form of secondary steel elements (probably lightweight box sections or cold-rolled steel) which would create a framework within the existing structure. This could help not only in the short-term, but could help to provide confidence for the continued use of RAAC structures in the long term as well.
For Help with RAAC Get In Touch Today
For assistance or advice with RAAC, get in touch with PorthouseDean today. Our experienced team of structural engineers is perfectly placed and ready to provide guidance and preliminary advice on ways of managing this problem.
Help for RAAC is here!For more information about what our dedicated team can do to help, if you have further questions surrounding the topic of “what is RAAC and Why is it a Problem?” or if you have a particular project you need support with, get in touch today.
